LONG TERM EARTHQUAΚE PREDICTION ΙΝ VRANCEA REGION,ROMANIA,BASED ΟΝ Α ΤΙΜΕ PREDICTABLE MODEL Panagiotopoulos,D.G. Geophysical Laboratory,Aristotle University of Thessaloniki,GR­ 54006 Thessaloniki,Macedonia,GREECE Α Β S Τ R Α C Τ Οη the basis of instrumental and historical information οη strong intermediate-deptll main shocks which occurred in the Vrancea region of the Romanian territory, four repeat times (five main shocks) have been determined. ΒΥ using these repeat tirnes, the following relation was obtained: 10gTt=0. 10M +0. 31Μ +a m1n ρ where Τ is the repeat time, measured in years, Μ is the magnitude of the preceding rnain shock, Μ is the maghitude of the srnallest earthquake considered and ,,~'.~ is a constant which corresponds to the source and equals to -1.45. Οη the basis of this relation the probability of occurrence of a new strong earthquake in Vrancea region during the next decades has been calculated. It was found that this probability,for the occurrence of a main shock with magnitude M~7.0, is less than 0.40 for the next two decades and more than 0.70 for the next three decades or so. ΜΑΚΡΟΠΡΟΘΕΣΜΗ ΠΡΟΓΝΩΣΗ ΣΕΙΣΜΏΝ ΣΤΗΝ ΠΕΡΙΟΧΗ VRANCEA ΤΗΣ ΡΟΥΜΑΝΙΑΣ ΒΑΣΙΣΜΕΝΗ ΣΤΟ ΜΟΝΤΕΛΟ ΠΡΟΓΝΩΣΗΣ ΤΟΥ ΧΡΟΝΟΥ ΤΩΝ ΙΣΧΥΡΩΝ ΣΕΙΣΜΏΝ παναγιωτόπουλος,Δ.Γ. Π Ε Ρ Ι Λ Η Ψ Η Ιστορικά και ενόργανα δεδομένα ιοχυρών ενδιαμέσου βάθους σεισμών στην περιοχή Vrancea της Ρουμανί.ας χρησημοποιήθηκαν για να δειχθεί ότι ο χρόνος ΤΙ (σε έτη) μεταξό δόο ισχυρών σειομών δίνεται από τη σχέση: LogT = 0.10M + 0.31Μ + a t min ρ όπου MjJI;n ε ί να ι το επ ι φανε ι ακό μέγε θος του μ ι κρότερου κόρ ι ου σε ι σμου που θεωροόμε, Μ το μέγε θος του κόρ ι ου σε ι σμοό που προηγήθηκε και "α" μια &ταθερά που αντιστοιχεί στη πηγή και υπολογίσθηκε ιση με -1.45. Με βάση τη σχέση αυτή υπολογίστηκε οτι η πιθανότητα για τη γένεση ενός κόριου σεισμοό με μέγεθος M~7.0 είναι μικρότερη του 0.40 για τις δύο επόμενες δεκαετίες (1993-2012) και μεγαλύτερη του 0.70 για τις επόμενες τρείς δεκαετίες (1993-2022). 482 Ψηφιακή Βιβλιοθήκη Θεόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ. INTRODUCTION Wi th the purpose to estimate the long term probabilities for the generation of strong earthquakes οη sing1e fau1ts, the time dependent mode1 seems to be more plausible in cornparison to the slip predictable rnodel (Wesnousky et a1.,1984; Astiz and Kanarnori, 1984; Nishenko and Bu1and, 1987).This mode1 holds even if the seisrnic source inc1udes, in addition to the main fau1t where the characteristic earthquake is generated, other srna11er fau1ts where srna1ler shocks also occur (Papazachos, 1989). According to the tirne-predictab1e mode1, the tirne of occurrence of a future earthquake in a certain seismic source depends οη the size and the time of occurrence of the last earthquake in the source. Οη the other hand, according to the slip-predictab1e model, the size of a future earthquake depends οη the time e1apsed since the 1ast earthquake. It rneans that we can predict,in princip1e, the size of a future earthquake by the slip-predictab1e mode1 and the time of its occurrence by the tirne-predictab1e mode1 (Bufe et a1., 1977, Shirnazaki and Nakata, 1980) . Very recent1y, Papazachos (1988a,b; 1992) conc1uded that the time-predictab1e rnode1 ho1ds for the sha110w earthquakes in Greece and he proposed a re1ation of the form: logT= bM,nin+CMp+a (1 ) between the repeat time, Τ, the rnagnitude, Μ, of the preceding main shock and the rnagnitude, Μ., of the s'hιa11est main shock considered in a certain seismog~~ic source. Α strong positive correlation between logT and Μ and a weak negative correlation between logT and Mf' where Mf ls the magnitude of the fo11owing rnain shock, has been found. Ιη addition, he proposed another Μ ι relation between and Mf which in combination with equation (1) 1eads to a modJl that permits an estimation of the time and rnagnitude of the expected main shock in a certain seismogenic $ource. Ιη the present study, we try to test the effectiveness of the tirne predictable model at the seismogenic source of Vrancea region in Rornania with data from intermediate depth earthquakes and to deterrnine the probabilities for the occurrence of strong earthquakes for the next years in this region. SEISMOTECTONICS ΑΝΟ PREVIOUS WORK ΟΝ EARTHQUAΚE PREDICTION ΙΝ VRANCEA The Vrancea sei~rnogenic area is located near the Romanian Carpathian Arc Bend (between the pa.rallels of 45.3"Ν and 46.0"Ν and between the meridians of 26.0 0Ε and 27.00Ε). The interrnediate depth earthquake generating stress systern and the subduction proccess under the Eastern Carpathians have been described by various researchers (Radu and Purcaru, 1964; Roman, 1970,1973ί Constantinescu, 1978; Morelli et al., 1975; Purcaru, 1979; Marza, 1982i Oncescu,1984i). This system occupies an area between the Eastern European major plate, the Moesian minor plate and 483 Ψηφιακή Βιβλιοθήκη Θεόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ. Transylvania which behaves as an intra-Alpine minor plate. Most οί the observed seismic activity is in the depth οί 80­ 150 krn (the maximum depth is about 200 krn). The strongest intermediate depth earthquakes have magnitudes Μ -7.5. The seismic activity at normal depth is much less significant than the main activity from intermediate depths but seems to be connected with the intermediate depth activity. Οη the other hand, there is ηο volcanic activity in the present day in this region. The Vrancea region is isolated from Alpine-Hima1ayan seismic belt and the aforementioned characteristics make it difficult to explain the cause of the intermediate depth earthquakes in terms of a typical subduction in a collision process. Apostol at a1. (1985} suggested that the Vrancea region is a post-subduction area in the present time with a dead slab.Their model suggested a compression in Lower Miocene with subduction of oceanic crust and consumption of the oceanic crust in Neogene with associated volcanism. 25" EASΤERN CARPATHIANS ΑΝΟ VRANCEA REGION ΙΝ ROMANIA Μ. MAJN ~ORE~r{lC~S SHOC,KS. MΤERIII:ICKS S.ει-6.4 Ο 6,!>-7.:;. ο __;;:o===-_.3όOkm Fig.1. The seismogenic source of Vrancea region in Romania with the epicenters of intermediate depth earthquakes and basic geologica1 information (modified from Morelli et al., 1975) . Taking into account the characteristics of Vrancea seismic region several researchers tried to apply some ideas to predict the seismic activity. Purcaru (1974,1979) attempted to describe the recurrence of intermediate depth earthquakes in Vrancea by a superposition of quasi-cycles. Οη the other hand, the investigation of seismicity revealed severa1 precursors: seismic gaps, changes of seismicity rates, b-value fluctuations etc., which all together prove that quiescence patterns work very well for the Romanian intermediate depth earthquakes (Apostol et al., 1985). Several other methods (temporal variation of focal mechanism, asperity distribution and application of the percolation theory to earthquakes, investigations οη the energy release etc.) have been applied by other researchers (Radu et 484 Ψηφιακή Βιβλιοθήκη Θεόφραστος - Τμήμα Γεωλογίας. Α.Π.Θ. al., 1984; Radu and Oncescu, 1985; Trifu and Radulian, 1989; Ardeleanu and Popescu, 1990; Ardeleanu and 5rnalbergher, 1990; Marza, 1991ί Trifu and Radulian, 1991; Radulian and Trifu, 1991 ) to investigate prernonitory seisrnicity in Vrancea region. DΑΤΑ ΑΝD ΜΕΤΗΟΟ The foci οί interrnediate depth earthquakes in Vrancea region forrn a well-defined seisrnic volume situated beneath the Carpathian arc in Romania territory, dipping down to 200 km depth (Oncescu, 1984 ί Trifu and Radulian, 1989 ί Trifu, 1990). This seismogenic source is shown in fig.(l} along with the epicenters οί the cornplete data οί the interrnediate depth earthquakes which are used in this study and some basic geological inforrnation rnodified frorn Morelli et al., (1975). Black circles show epicentres οί the main shocks. Open circles show epicentres of the foreshocks and aftershocks in the broad sense, that is, earthquakes which may occur υρ to several years before or after the rnain shock, respectively. We use the terrns "aftershock" and "foreshock" in their broad sense because we want a rnodel which can predict the main shocks in the seisrnogenic source οί Vrancea, that is, the strong earthquakes which occur at the beginning and the end of each seisrnic cycle and not srnaller earthquakes which occur during the preseisrnic and postseisrnic activations. Ιπ particular, as foreshocks and aftershocks have been considered the earthquakes which had preceded or followed the main shocks with magnitudes 5.5-5.7, 5.8-6.0, 6.1-7.0 and larger than 7.0, υρ to 6,7,9 and 13.5 years, respectively (Papazachos, 1992). Inforrnation οη the surface wave rnagnitudes and οη the epicentres οί the earthquakes plotted in fig. (1) were received from the catalogues οί Karnik, (1968, 1971) and Shebalin et al., (1974) for the period 1880-1904, οί Comninakis and Papazachos, (1978) for the period 1908-1948 and frorn the rnonthly bulletins οί the National Earthquake Information Center οί Geological Survey οί U.5. Department οί the Interior for the period 1977­ 1990. Information οπ main shocks used in the present study is given in Table (1). The first column οί this table gives the narne οί the seisrnogenic source, while the second one gives the year since when the data are complete for a certain minimum magnitude, which is written next to it. It means that the data are complete for earthquakes with magnitudes M~6.5 during 1880-1991 and for Μ >5.8 during 1901-1991. This completeness has been determined o~the basis οί information given in the catalogues.